The present invention relates to a method and device for detecting and locating irregularities in the dielectric around a conductor, wherein the conductor is substantially elongate. The irregularities in the dielectric around a conductor can cause so-called partial discharges. These can occur for instance in the dielectric material situated between a core and an earth sheath of a cable. The partial discharges can result in loss of quality of the cable. Partial discharges are moreover often the precursor of complete discharges, or breakdown, whereby the cable becomes defective.
Known from the international patent application WO 94/10579 is a method and device for detecting partial discharges occurring in a cable, in which the earth sheath of the cable has a helical structure, wherein on one side of the cable at least one detection coil is arranged around the cable for detecting electromagnetic voltage changes which displace along the cable. The voltage changes, which are generated by possible partial discharges and which are propagated in both directions of the conductor from the position of the partial discharge, are detected and analyzed. Use is made herein of so-called end reflections of the voltage changes on the outer ends of the cable. The detection coil will first detect the direct voltage change which is transmitted directly from the position of the partial discharge to the detection coil. The detection coil will then detect the indirect voltage changes which are transmitted from the position of partial discharge to the detection coil via a reflection against an end of the cable. On the basis of the measured time difference between detection of the direct voltage change caused by a partial discharge and detection of the indirect voltage change caused by a partial discharge and reflected against the end of the cable, and also on the basis of the fact that the propagation speed of the voltage change is known, the path length difference between the direct and indirect pulse can be determined. The position of the partial discharge can be determined in accurate manner from the computed path length difference and the already known total length of the cable.
This method does however have a number of significant drawbacks. Since the indirect voltage change often arrives at the detector in the xe2x80x9ctailxe2x80x9d of the direct voltage change and moreover the indirect voltage change often has a smaller amplitude than the direct voltage change, the indirect voltage change can often not be recognized, or only with great difficulty. One consequence of this is that analysis of the measurements cannot be automated and that the analysis must be performed by highly qualified and highly trained personnel. This, however, entails high cost.
A method is sought wherein these drawbacks can be reduced or obviated.
The object of the present invention is to provide a method and device with which, either in manual or automated manner, the positions of irregularities in conductors can be simply determined. This object is achieved in the manner described below.
Instead of detecting the direct and indirect voltage changes on one side of the conductor, as described above, according to the present invention measuring takes place on both sides of the conductor. Herein only the direct voltage changes arriving on both sides are analyzed. The indirect, i.e. Reflected, voltage changes arriving on both sides are not taken into consideration. However, in order to be able to determine the differences in arrival times of the voltage changes caused by one and the same irregularity on both sides of the cable, a very accurate time registration is required. By means of this time registration the measured signals can be provided with a so-called time label, with which the progression of the signal in time can be determined. The signal provided with a time label can be stored on a random medium or be transmitted via communication means to a central collection unit. On the basis of the time difference between the moment at which the direct voltage change arrives on the one side and the moment at which the direct voltage change arrives on the other side and on the basis of the fact that the propagation speed of the voltage change is known, the path length difference, i.e. the difference in the distance covered by the two voltage changes, can be determined. On the basis of this determined path length difference and the already known total length of the cable, the location or position of said irregularities can be determined.
According to a preferred embodiment of the present invention, time registration is performed by coupling time and position data received via a plurality of satellites to the measured voltage changes. The time data received per determined detection position and transmitted via satellites are herein corrected for the exact detection position which follows from the data transmitted via the satellite. This correction is necessary in order to obtain the required accuracy in time, since the voltage changes displace practically at the speed of light and the difference in arrival time of the voltage changes is thereby very small (in the order of magnitude of xc2xd to many tens of microseconds).
According to another preferred embodiment of the present invention, time registration is carried out by means of very accurate clocks, for instance crystal clocks, which must be calibrated prior to the measurements. These clocks supply the time registration which is required at each detector in order to provide the detected voltage changes with a correct time label.
According to yet another embodiment of the present invention, branched cables can be used instead of single cables. A plurality of (preferably all) branches of the cable are herein provided with voltage detection means. The positions of the irregularities can be determined in a manner wholly analogous to that described above.